To open an app that isn’t allowed to open you can manually override the Gatekeeper settings in Security & Privacy preferences by choosing open from the shortcuts menu. The app is saved as an exception to the settings and you can open it again in the future just like any authorized app, by double-clicking it.

In the Finder, locate the app you want to open. Most apps can be found in the Applications folder.

Press the Control-key and click the app.

Choose Open from the shortcut menu.

Click Open.

For more information about protecting your Mac from harmful software, see this help topic: